Coating materials may be used for visual or construction physics-related reasons to coat facades and other construction surfaces. But the fact that the cleaning of the surfaces coated with such coating materials is often difficult and costly has turned out to be a problem. For this reason, efforts have been made for a long time to look for ways of creating self-cleaning designs of the surfaces of the areas coated with such coating materials.
In conjunction with the implementation of self-cleaning surfaces, coating materials on the basis of mineral paints, which are subject to constant decomposition on the surface, have been known for a long time. Due to this process, also known as “chalking,” the surfaces regenerate constantly and dirt adhering to them is removed in the process of the decomposing coating. But these surfaces have the disadvantage that the irregular and, in some cases, complete decomposition of exposed areas results in patchy surfaces and that the chalking not only causes the rapid decomposition of the coating and, accordingly, shortened restoration intervals, but that also, upon contact with the coated surface, the materials touching it (e.g. clothing) are soiled with the decomposition products of the coating.
In addition to the self-cleaning coating materials based on mineral paints, self-cleaning coating materials having the form of silicone resin paints have also become known. Silicone resin paints form a hydrophobic surface which is barely moistened by precipitation water. Dirt particles are washed off the surface by the water draining off. But with such coating materials, it has proven to be a problem that the hydrophobicity required for the achievement of the self-cleaning properties is not fully developed until after weathering has taken place for several months, for that is the time it takes for the rain to completely wash out the water-soluble components contained in the coating material.
But that leads to the possibility of increased soiling taking place within the first few months of the completion of the coating. Such a tendency for soiling is particularly observed following a long periods of dry spells, after which large quantities of dirt particles and pollutants are in the atmosphere and are absorbed by the precipitation water. The dirt particles are deposited on the surfaces capable of being moistened and result in an impairment of the appearance of the facades and other soiled surfaces onto which they are deposited.
In WO 00/39049, the creation of self-cleaning surfaces using coating materials of the kind described above is described. The coating materials described in this publication contain fillers exhibiting an at least bimodal particle size distribution, using, on the one hand, particles having a particle size of at least 5 μm and, on the other hand, particles having a particle size of max. 3 μm. The use of such coating materials has the effect that soiled rain water runs off the surface of an object and that dust particles deposited on the surface are pulled along by the water drops rolling off. In addition, the use of the coating materials described in the publication named above results in a permanently dry facade due to the running off of the rain water achieved by the use of the special fillers. This enables the prevention of damage due to moisture, especially on the weather sides of the facades. Furthermore, the achieved drying of the facades deprives microorganisms of an important element of their basic living conditions, namely water, allowing facade surfaces obtained by using the known coating materials to be protected from infestation with fungi, algae, lichens etc. in a natural manner, i.e. without the addition of biocides. A coating material comparable to the coating materials known from WO 00/39049 is also described in EP 0 772 514 B1. With the coating material described in this publication, a self-cleaning surface is obtained by producing a surface structure having elevations spaced apart 5 to 200 μm, using hydrophobic polymers or permanently hydrophobized materials, while making sure that the elevations are not capable of being detached by water or water containing detergents. This microstructure of the surface produces a self-cleaning property which is due to a so-called super hydrophobicity, caused by special microstructures. This property has become known as the “Lotus effect.”
According to the publication named above, the desired surface structure can be obtained by an after-treatment of the surface, such as e.g. stamping, etching, milling or covering of the surfaces with dust, with it being required to ensure, in every case, that the thus obtained elevations cannot be removed by water or water containing detergents. The self-cleaning surface structure described in the publications mentioned above may be obtained on smooth surfaces as well as by using special sprays.
In WO 00/06633, coating materials for plastics based on inorganic binding agents and fillers are described. The coating materials set forth in the known publication may additionally contain photocatalytically active agents for self-cleaning by means of decomposition of organic soiling and superhydrophilics, while the binding agents used in the manufacture of the known materials are stable to the photocatalytic effect of the photocatalytically active agents.
In EP 0 916 411 A1, a coated product with a first coating layer and a second coating layer containing a photocatalytically active agent is described. The combination of two layers described in the above-mentioned publication produces a photocatalytically self-cleaning surface which is not even degraded by the photocatalytic action.
When using the coating materials known from EP 0 772 514 B1 and WO 00/39049, such as e.g. the sprays described for smooth surfaces, it turned out, however, that the originally observed outstanding self-cleaning properties can not be maintained permanently, even when, as emphasized in EP 0 772 514 B1, it is carefully made sure that the microstructure cannot be detached by water or water containing detergents.